Viking labeled release (LR) experiment

One of three biology experiments carried aboard each of the Viking
landers.

Originally known as "Gulliver";1 the labeled release (LR) experiment
was developed by Gilbert Levin and designed
to detect carbon dioxide released
by microorganisms as a result of their metabolic activity.2 On
Earth, this would be classed as a respiration experiment. A sample of soil
was placed inside a culture chamber and
a broth of 7 organic nutrients (formate, glycolate, glycine,
D-alanine, L-alanine, D-lactate, and L-lactate),
labeled with radioactive carbon-14, allowed to drip onto it. If microbes
were present in the sample, it was assumed they would metabolize the organic
compounds in the nutrient and release radioactive carbon dioxide which would
be trapped on a chemically coated film at the window of a Geiger counter.
The L*R experiment had the virtue of being able to detect growth, as well
as metabolism, since the rate of carbon dioxide production would increase
exponentially with a growing culture. Like the Viking
gas exchange (GEX) experiment, however, it assumed that Martian microbes
would be (a) activated by liquid water (which might not be true since any
organisms on Mars might have adapted to a completely
waterless environment), and (b) able to metabolize the same organic nutrients
as terrestrial organisms (whereas, in fact, such Earthly foodstuffs might
be toxic to life based on a different biochemistry). These drawbacks were
avoided by the Viking pyrolytic release (PR) experiment.